According to page 90 of the datasheet [1], AIN_BUFP is bit 6 and
AIN_BUFM is bit 5 of the CONFIG_0 -> CONFIG_7 registers.

Fix the mask used for setting these bits.

[1]: https://www.analog.com/media/en/technical-documentation/data-sheets/ad7124-8.pdf

Fixes: 0eaecea6 ("iio: adc: ad7124: Add buffered input support")
Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Link: https://lore.kernel.org/r/20220112200036.694490-1-cosmin.tanislav@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>